A great deal of interest has been focused on UV LEDs and lasers, in particular those that emit light in the blue and deep ultraviolet (UV) wavelengths. These opto-electronic devices may be capable of being incorporated into various applications, including solid-state lighting, biochemical detection, high-density data storage, and the like.
A modern opto-electronic device, such as a UV LED, typically includes three major components: an electron supply layer (e.g., a n-type semiconductor layer), a hole supply layer (e.g., a p-type semiconductor layer), and a light generating structure formed between the electron supply layer and the hole supply layer. These UV LEDs are mainly fabricated from group III nitride heterostructures. Typically, a high level of p-type doping is necessary with the group III nitride materials used in these heterostructures in order to have efficient LED operation. Achieving the necessary p-type doping for efficient operation is a challenge due to high ionization energy of acceptor impurities in these group III nitride materials.
Several approaches have been used to attain the necessary p-type doping in these group III nitride based UV LEDs. In one approach, an additional SiO2/semiconductor interface is used at the part of the UV LED where the hole accumulation layer is formed within a p-type layer. With this approach, the holes need to tunnel through the SiO2/semiconductor interface to reach the light generating structure. The hole accumulation layer in this design can help increase the hole concentration, but the concentration of holes tunneling through the SiO2/semiconductor interface is significantly lower than that in the hole accumulation layer. Also, adding the SiO2/semiconductor interface in this design substantially increases the turn-on voltage of the UV LED device.
In another approach, a tunnel junction formed between two group III nitride layers is used for hole injection into the light generating structure. In this approach, a high electron concentration in the top group III nitride layer causes carrier tunneling through the tunnel junction and reduces the lateral spreading resistance. However, this design does not allow for significant improvement in the hole injection into the light generating structure. In a third approach, a hole acceleration layer is used with the UV LED device structure. In particular, a group III nitride layer and a group III nitride barrier form a region with strong electric field enhancing the hole emission over the barrier. This approach results in excessive voltage drop across the additional barrier without any significant increase in the hole injection into the light generating structure. All of these design approaches for group III nitride based UV LEDs lack the capability to enable high carrier concentration in the light generating structure due to high ionization energy of the doped group III nitride materials.